It is known in the art relating to lean-burn engines, that the oxygen concentration in the intake manifold has an impact on combustion and emissions by changing constituents of the cylinder charge. In order to estimate the oxygen concentration in the intake manifold, it is required to measure or estimate air flow, EGR flow, and the flow into the cylinder. In a standard engine, air flow is measured by a Mass Air Flow (MAF) sensor at the throttle body.
A conventional, low-cost MAF sensor is insensitive to the flow direction, and it usually works well since the intake manifold pressure is controlled by throttle to be less than ambient pressure in most engine operating conditions, and as a result, the flow is one directional from the throttle body to the intake manifold. However, under unthrottled operating conditions, the intake manifold pressure becomes nearly ambient, and resonance waves can occur through intake systems. Those waves travel through the intake manifold, and may cause reverse flows upstream of the intake manifold where the MAF sensor is typically located.
It has been shown that a conventional, directionally insensitive MAF sensor significantly overestimates air flow due to those reverse flows, especially at low air flow as seen in FIG. 1. In the figure, the solid diagonal line 10 represents mass air flow into the engine measured at a calibrated orifice upstream of the engine intake. The separate squares 12 show individual readings of a hot wire MAF sensor at air flows of 0–200 kg/h. It is seen that below about 100 kg/h, the intake manifold pressure sensor readings are increasingly high. It has also been noted that even specially designed MAF sensors, which are only sensitive to forward directional flow, do not improve the accuracy much at low air flow, since air flow becomes unsteady as indicated by large cycle-to-cycle variations and frequency doubling.
Also, given a mass air flow, EGR flow can be estimated by subtracting the mass air flow from the mass flow into the cylinder based on an assumption that intake manifold dynamics are fast enough to ignore, which is true when the engine is operating under unthrottled operating conditions. For an engine equipped with conventional intake and exhaust valves, the mass flow into the cylinder can be estimated based on the volumetric efficiency, the engine speed, and the intake temperature, etc. The volumetric efficiency depends on operating conditions, and requires extensive calibrations for accurate estimation. Thus, the estimated EGR flow using this method would contain uncertainty as long as measured mass air flow and/or estimated mass flow into the cylinder are inaccurate.